Itch.io has responded, and "topping up" previous purchase isn't something that is currently supported. Currently, the only way to do it would be to ask for a refund, and then re-purchase with a higher value. If you want to do this, send me an e-mail to meyaoigames@gmail.com with your itch.io account e-mail address (I'll relay to itch.io regarding the refund).